How many twins?
I don't have the DVD of this (yet) and I haven't seen this great film since the aforementioned showing on BBC2 in the late '70s.
However, lots of bits from this film stick in the memory and one of them was that there were a lot of twins in the cast - but how many?
PS: Using twins is a great way to get a special effect on the cheap - even that CGI-fest "Terminator 2" used two sets of twins.
If you could see what I've seen with your eyes!